This invention is directed to an improved digit display driving circuit, and in particular to a liquid crystal digit display driving circuit. Within the past few years, electronic instruments having digital displays have utilized liquid crystals in view of their minimum power consumption, their suitability for use with miniaturized circuitry and their passive display characteristics.
Heretofore, liquid crystal digital display drive circuits have effected a static display in one of several manners. A first type of drive circuit utilizes a register for storing parallel display signals therein, which display signals are applied to the display cells for a predetermined period. A second type of drive circuit utilizes a memory for each display digit, and applies a time division multiplexing signal to each memory to energize the display cells for a short period of time. A third type of drive circuit stores the display driving signals in series in a shift register and directly drives the display cells for a predetermined period determined by the shift register.
In the first two types of circuits, the number of output terminals for the circuitry effecting the logic operation is increased, increasing the number of input terminals of the circuit for driving the display cell, when the display is to be driven by an alternating current drive. The additional input terminals of the display cell increase the complexity of the wiring of the display drive circuit rendering more difficult the forming of same in a circuit chip by large scale integrated circuit techniques.
When the third type of drive circuit is adopted, namely, utilizing a shift register, since the signals applied to the display cell are series signals, the drive circuitry is simplified and so is the manner in which the integrated circuit is coupled thereto. However, since display signals are directly applied to the display cell by the shift register, the period of transmission of the display signals becoms related to the number of digits to be driven. If the frequency of the clock signal applied to the shift register is low, and if the transmission period is lengthy while the rate of the transmission period to the display period is small, the display cell begins to flicker thereby causing the clarity of the display to deteriorate. Also, as noted above with respect to the other drive circuits for effecting a static drive of the display cells, the number of terminals of the digital display corresponds to the number of each of the segments forming the display digits, thereby complicating the fabrication of devices utilizing such display cells, during manufacture. Moreover, in order to increase the operating life of the liquid crystal display cells, each display cell must be driven by an alternating current, the aforementioned driving circuits thereby requiring an EXCLUSIVE OR gate or other type of alternating drive circuit element for each segment forming each of the display digits.